The “toll plaza effect” in a vehicular traffic system refers to the challenge involved in accurately estimating traffic conditions on road segments that contain a toll plaza. The challenge arises from the fact that a probe vehicle traveling on such a road segment may report a plurality of different speed distributions depending on its precise location relative to the toll plaza and the particular time interval (epoch) during which the probe vehicle transmits its report. For example, the probe vehicle may report a high speed (e.g., 100 kph) as the vehicle approaches or leaves the toll plaza, but a very low speed (e.g., 0 kph) as the vehicle slows or stops during payment of the toll.
If, during a first time interval, the probe vehicle transmits only low speed observations to a vehicular traffic system, the vehicular traffic system may mistakenly interpret the low speeds as indicative of congestion and report such a traffic condition to a user (e.g., by coloring a road segment on a map as red). However, in actuality, the low speeds reported by the probe vehicle during the first time interval may be due merely to the slowing of the probe vehicle during payment of a toll—not to traffic congestion per se. In contrast, during a second time interval, the probe vehicle may transmit only high-speed observations to the vehicular traffic system and, on this basis, the vehicular traffic system may determine that traffic is not congested and report such a finding to the user (e.g., by coloring the road segment on a map as green).
Since the determination of “congestion” or “no congestion” may vary according to the particular time interval during which the probe reports are transmitted from the probe vehicle, an undesirable condition known as “thrashing” may result. During thrashing, the traffic conditions for a given road segment may flip back and forth in rapid succession. For example, the coloring of the road segment on a map may flip back and forth between red (e.g., a color indicative of heavy congestion) and green (e.g., a color indicative of little or no congestion). As a result, there may be an increase in congestion messages for certain traffic feeds (e.g., traffic feeds whose specification includes cancel messages that cancel a previously reported congestion level if the current congestion level changes).